Apparatus and methods of continuous digesting

ABSTRACT

METHODS FOR THE CONTINUOUS PULPING OF A COMPACTED MASS OF WOOD CHIPS IN AN UPRIGHT ELONGATED REACTION VESSEL HAVING MEANS FOR FEEDING WOOD CHIPS SUBSTANTIALLY CONTINUOUSLY INTO THE INLET END OF THE VESSEL AND DISCHARGING THEM SUBSTANTIALLY CONTINUOULY FROM THE OPPOSITE OUTLET END OF THE VESSEL, THE MASS OF WOOD CHIPS BEING ADVANCED THROUGH THE VESSEL FOR TREATMENT IN LIQUID SUBMERGENCE BY ADVANCING LIQUID THROUGH A PORTION OF THE SUBMERGED CHIP MASS ADJACENT THE INLET END OF THE VESSEL. IN ONE EMBODIMENT THIS IS ACCOMPLISHED BY RECIRCULATING FROM ONE INTERMEDIATE STRAINER BACK TO THE ENTRANCE END OF THE VESSEL, AT A TURBULENT LIQUID FLOW RATE DIRECTED GENERALLY ALONG THE LENGTH OF THE CHIP MASS FOR A SUBSTANTIAL DISTANCE OF AT LEAST ABOUT ONE-HALF THE CROSS SECTION DIMENSION OF THE MASS, SUCH PROVIDING A WOOD CHIP PROPELLING FORCE EFFECTIVE TO ADVANCE THE MASS OF WOOD CHIPS CONTINUOUSLY   THROUGHOUT THE LENGTH OF THE VESSEL IN ANOTHER EMBODIMENT, IT IS ACCOMPLISHED BY GENERATING LIQUID PROPELLING PULSES, PREFERABLY BY UTILIZING AN ACCUMULATOR AT ONE END OF THE VESSEL AND A CYCLING TIMER FOR OPERATING IT, SUCH PROPELLING PULSES BEING EFFECTIVE TO CAUSE THE RATE OF MOVEMENT OF THE SUBMERGENCE LIQUID RELATIVE TO THE WOOD CHIPS IN THE MASS TO VARY CYCLICALLY TO PRODUCE A CYCLIC ADVANCING MOVEMENT OF THE LIQUID AT A RATE AT LEAST NOT LESS THAN THAT OF THE ALTERNATING INTERVENING MOVEMENT OF THE LIQUID, SUCH ADVANCING MOVEMENT OF THE LIQUID BEING EFFECTIVE TO ADVANCE THE WOOD CHIPS THROUGHOUT THE LENGTH OF THE VESSEL FROM THE INPUT TO THE OUTPUT END THEREOF.

Oct 24 '4 J. P. RICH 3,700,548l

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United States Patent Oice 3,700,548 Patented Oct. 24, 1972 3,700,548 APPARATUS AND METHODS OF CONTINUOUS DIGESTING John Parker Rich, Nashua, and Lawrence Allan Carlsmith, Amherst, N.H., assignors to Improved Machinery, Inc., Nashua, N.H.

Filed Apr. 28, 1971, Ser. No. 138,295 Int. Cl. D21c 3/26 U.S. Cl. 162--17 3 Claims ABSTRACT OF THE DISCLOSURE IMethods for the continuous pulping of a compacted mass of wood chips in an upright elongated reaction vessel having means for lvfeeding wood chips substantially continuously into the inlet end of the vessel and discharging them substantially continuously from the opposite outlet end of the vessel, the mass of wood chips being advanced through the vessel for treatment in liquid submergence by advancing liquid through a portion of the submerged chip mass adjacent the inlet end of the vessel. In one embodiment this is accomplished by recirculating from one intermediate strainer back to the entrance end of the vessel, at a turbulent liquid ow rate directed generally along the length of the chip mass for a substantial distance of at least about one-half the cross section dimension of the mass, such providing a wood chip propelling force effective to advance the mass of wood chips continuously throughout the length of the vessel. In another embodiment, it is accomplished by generating liquid propelling pulses, preferably by utilizing an accumulator at one end of the vessel and a cycling timer for operating it, such propelling pulses being effective to cause the rate of movement of the submergence liquid relative to the Wood chips in the mass to vary cyclically to produce a cyclic advancing movement of the liquid at a rate at least not less than that of the alternating intervening movement of the liquid, such advancing movement of the liquid being effective to advance the wood chips throughout the length of the vessel from the input to the output end thereof.

This is a continuation of our application Ser. No. 806-,- 157, filed Mar. 11, 1969 (now abandoned), which was, in turn, a continuation-impart of our applications Ser. No. 508,531, filed Nov. 18, 1965 and Ser. No. 548,225, filed May 6, 1966 (both now abandoned) and relates to the continuous digesting of wood chips in an upright elongated digester lvessel, and more particularly to novel methods for continuously advancing a compacted mass of wood chips throughout the length of such a vessel.

Upflow digesters of the type shown and described in our earlier U.S. Pat. No. 2,878,116 and Pulp and Paper, New Continuous Digester of September 1959 have proved to be highly successful because of their improved thermal stability over downow digesters and their unique ability over conventional downow digesters to provide chip drainage, preceded, if desired, by high temperature countercurrent washing. A short stroke bottom piston is utilized in these upow digesters to provide the force both for creating and maintaining the lower end of the compacted mass of wood chips and advancing the entire mass upwardly throughout the length of the digester in spite of the presence of multiple strainers and countercurrent flow and drainage zones and the like.

An advantage of upflow digesters is their capability of providing a drainage zone at the top, for draining spent liquor from the cooked chips prior to further treatment, in order to increase the efliciency of chemical recovery. However, with upllow digesters, the substantial weight of the drained chips in suoh a drainage zone opposes the advance of the chip mass lower in the digester.

Too, in multiple zone digesters, a number of strainers are provided around the perimeter of the digester at various locations therealong for withdrawing liquor in order to accomplish heating, treating or washing of the chip mass in the various zones. These also have the adverse effect of retarding the advance of the compacted chip mass, as chips tend to cling to the strainer surfaces because of the pressure of the liquor being withdrawn.

Furthermore, the countercurrent flow zones used to advantage in such multiple zone digesters for carrying out a number of sequential process steps such as heating and washing produce retarding forces on the advancing chip mass.

Because of these factors, the forces which must be created in order to oppose the resistance within the vessel must be quite high if the chip mass is to be advanced. For this reason, the short stroke bottom piston heretofore utilized in commercial upflow digesters has been relatively massive and hence a major factor in their cost and mechanical complication, While the application of the high forces applied at the lower end of the chip mass has tended to compact portions of the chip mass thereabove to an undue degree which caused undesirable variations in the pulping reaction.

Accordingly, it is a major object of the present invention to provide novel methods for propelling the compacted mass of wood chips throughout the length of the digester vessel while maintaining a much more uniform com paction of the chips in the mass than has heretofore been possible.

It is another object of the present invention to provide novel methods for eliminating the expense, in such digesters, of a short stroke bottom piston, thereby greatly reducing the cost, while retaining all the advantages thereof.

It is still another object of this invention to provide novel methods in a downflow digester.

In general, the present invention accomplishes such oibjects by advancing liquid through a portion of the submerged columnar mass of compacted wood chips adjacent the inlet end thereof at a liquid flow rate effective to create a force which will propel the entire columnar mass of wood chips extending throughout the length of the digester, even if there be countercurrent liquid flow through portions of the wood chip mass, and, in an upow digester, even if there be a Wood chip drainage zone at the upper end of the chip mass.

The unique results of the invention are accomplished either by providing a continuous liquid flow or, more preferably, by providing liquid propelling pulses. In the former case, the results of the invention are made possible because, as is known from hydraulic theory, the rate of uid llow resistance increases much more rapidly with increased vllow velocity within the turbulent flow region than it does within the laminar ilow region. The presest invention is based on the discovery that turbulent flow occurs within a compacted chip mass at far lower flow rates than had heretofore been assumed. In the case of liquid propelling pulses as produced by a liquid propelling pulse generating means, such as a cyclically valved accumulator, effective to cause the rate of movement of the submergence liquid relatively to the wood chips in at least a portion of said mass to vary cyclically, the invention provides an average net propelling pulse force acting in the submerged chip mass which is highly effective to advance the wood chips throughout the length of the wood chip column from the input to the output end thereof, in either upflow or downflow vessels.

In upliow vessels, such liquid flow, when appropriately controlled by a sensor for the lower end of the chip mass, also serves to position the lower endof the chip mass, so that the level of compacted chips deposited thereon may be kept uniform.

Unexpectedly, we have found that liquid propulsion of the entire columnar chip mass can be achieved using liquid flows, whether continuous or pulsed, within a propelling zone therein at ilow rates not too high to be provided by conventional pumps and strainers, yet high enough to be within the turbulent ow region. These flows create suitably high flow resistance within a propelling zone of practical length which may be located at the lowermost end portion only of the columnar chip mass.

For the purpose of fully explaining preferred embodiments of the invention, reference is now made to the following specification, together with the drawings where- 1n:

IFIGS. 1 through 3 are diagramamtic side elevations of a. rst, second and third embodiment of apparatus suitable for carrying out the methods of the invention; and

FIG. 4 is a typical graph of increasing tluid resistance with increasing fluid flow velocities through the laminar ow region into the turbulent flow region.

'Referring to FIG. 1, the pressurized vessel 12 therein shown is generally cylindrical of the order of ten feet or greater in diameter and one hundred feet in length so that it has a length to diameter ratio of at least one to four. It is believed to be important that vessel 12, although of generally uniform cross sectional area, have a slight taper to provide a gradually upwardly increasing cross sectional area, a taper of about inch of diameter per foot of length being suitable, but preferably not exceeding about l `inch of diameter per foot of length. A wood chip-liquid infeed is provided at the lower entrance end of vessel 12 in the form of a tangential inlet 16 to which is fed, by means of a pump 18, preferably of the type shown and described in Pat. No. 2,908,226, a wood chip-liquid mixture from a supply tank 20. At the upper end of the vessel 12 is provided a discharger for the pulped wood chips in the form of a scraper 22 which discharges drained pulped wood chips through discharge outlet 24 to a discharge mechanism at the lower end of downow pipe (not shown) such as is described in Pat. No. 3,206,356.

Within vessel 12, generally along the central vertical axis thereof, are provided a series of three concentric injection pipes 41, 43 and 53 having outlets 42, 44 and S4, respectively, at successively higher levels. Also within vessel 12 a provided strainers 46 and 48, together with said outlets establishing a plurality of zones within the vessel by advancing liquid for either co-current or countercurrent ow within said zones as desired. For example, as shown in FIG. l, there is provided an impregnation and heating zone with superposed heating and cooking zones with co-current and countercurrent flow and a countercurrent washing zone with an uppermost draining zone, the liquid flows being shown by the single arrows and the wood chip movement by the double arrows. A liquid level control 50 is provided near the upper end of vessel 12 to establish the liquid level below scraper 22 as by operating valve 52 in upper strainer 48 to provide the chip drainage zone.

In accordance with the present invention, a liquid system is provided for advancing liquid upwardly through a lower portion only of vessel 12 for propelling the entire columnar chip mass upwardly through the vessel by high velocity liquid ow upwardly through said lower portion. Broadly, the apparatus components of such system include a lower propelling liquid strainer 30 spaced a substantial distance `above inlet pipe 16 and below scraper 24 defining a propelling zone therebelow and a recirculation pump 32, together with connecting pipes 34 and 38 and valve 53, for controlled recirculation of liquid to pump 18. As more fully described in co-pending patent application Ser. No. 543,363, a sensor 51 is provided near the lower end of vessel 12 to control the position of the lower end of the chip mass as by operating valve 53 to vary the flow of recirculating liquid and hence the rate of movement -of the wood chip mass.

As hereinafter discussed in detail, it is vital to the invention that the propelling zone be of substantial length at least about one-half the diameter or other cross sectional dimension of vessel 12 and be located at the lowermost end of the chip mass generally adjacent the lower end of the vessel and that certain minimum liquid llows well within the turbulent ow region be created therein for upward advance of the chip mass in accordance with the principles of the invention.

More specifically as to the methods of the invention and the continuous flow operation of the above described apparatus of FIG. l in accordance therewith, it has been established in the commercial operation of digesters of the type shown and described in Pat. No. 2,878,116, that the wood chips are present therein in the form of a columnar mass of contacting and compacted but discrete wood chips through which liquid can flow more or less independently of the wood chips in the mass but subject to a flow resistance due to the presence of the wood chips. Because the wood chips are present in the mass in discrete form but in contact with one another, the wood chips within the mass can be moved throughout the length of the vessel solely by pushing on the bottom end surface of the columnar mass within the vessel regardless of the direction of liquid flow therein. Such operation, in fact, occurs in digesters of the type shown and described in said Pat. No. 2,878,116, said Pulp and Paper article and in Pat. No. 3,061,007, the latter two showing and describing a digester in which the top portion of the chip mass extends out of submergence into a drainage zone above the surface of the liquid in the vessel, so that the weight of such portion is completely unsupported by the liquid. Within such digesters, however, the degree of wood chip compaction tends not to be uniform throughout the length of the chip column, so that the wood chip digestion is not as uniform as is desired. Furthermore, with digesters of the type described in said patents, the foraminous reciprocating piston member tends to be unduly massive, and also makes awkward the use of a center injection pipe, as is desirable with digester vessels of large diameter.

As pointed out above, it has been discovered with respect to the present invention that the compacted wood chips can be advanced throughout the entire length of the columnar mass maintained within the vessel by advancing liquid through an initial portion only of the chip mass at a liquid llow rate within the turbulent flow region. The propelling force so created is suiciently high t0 move the chip mass even if there be countercurrent liquid ow throughout other portions of the Wood chip mass and a wood chip drainage zone at the upper end of said mass. The ability to provide such a highly desirable multiple zone upflow system with its inherent economics in heating, chemical retention and chip drainage is a particularly important feature of the present invention. Also, since the forces needed at the lower inlet end of the wood chip column for carrying out chip accretion and compaction thereon are relatively small, such accretion and compaction may be accomplished either by the mechanical device of said patents, but with much less massive apparatus, or with the propulsion liquid approach hereof, but with much lower flow rates and propulsion column lengths. In a downow system, the action of gravity will serve to form a compacted chip mass.

The general concept of turbulent liquid ow is well understood in the hydraulic art, and is illustrated in FIG. 4. Thus, within the region of laminar flow, the resistance to flow is directly proportional to the ow velocity, whereas within the turbulent ow region it increases as the power (n) of flow (Q), where n is greater than unity. The resistance to ow created by the presence of the columnar mass of chips Within the propelling zone can be determined by measuring the difference (AP) between the uid pressure (P1) at the bottom of the propelling zone and the uid pressure (P2) at the top of the propelling zone. With fluid flow increasing from zero, the resistance (AP) will rise proportionally throughout the laminar ow region, and then rather abruptly change in a short transition region to a much more rapid rise as the ow enters the turbulent region. In this way, the beginning of the turbulent ow region for a specific chip mass can readily be determined so that a fluid flow well within, that is beyond the transition region, can readily be selected.

The propulsion factor (Fp) of the present invention is generally proportional to the upward liquid llow (Q') in gallons per minute per square foot of vessel cross-section area (raised to the power n), times the length (L) in feet of the lower portion of the columnar chip mass through which such flow occurs (the propelling zone) as approximately expressed in the following equation:

where nis greater than about 1.5, as occurs with turbulent flow, usually within the range of about 1.5 to 2.5. The factor Fp depends greatly on the porosity of the chip mass in the propelling zone and so varies depending on the chip size, compressibility and the amount of sawdust or slivers included. Thus, the value of propulsion factor (Fp) needed for a particular digester can be determined from the characteristics and dimensions of the chip mass zones maintained therein, the condition of such zones as to nonsubmergence or submegence and the forces created thereby because of factors such as the presence of strainers, chip weight and fluid ow forces co-current or countercurrent.

In practice, with a wood chip mass of particularly slow drainage characteristics such as Douglas r sawmill waste including sawdust and slivers and with a relatively short drainage zone, the propulsion factor (Fp) might be quite low, but with a value in excess of about 500 in order to be eifective according to the invention. Higher values are needed with the preferred longer drainage zones and with freer chip masses of the order of about 100 times higher in some instances.

The length of the propelling zone in relation to the diameter or other transverse dimension of the chip mass is important in maintaining flow uniformity and in preventing channelling, that is, the opening of large liquid flow channels through the chip mass as tends to occur between closely spaced uid inlets and outlets. It is preferred that the propelling zone be of at least the same order or somewhat greater than that of the transverse dimension of the chip mass, but certainly not less than about one-half of said dimension. Too, with short propelling zones, the necessary liquid flow velocity tends to become unduly high.

It will be understood that the direction of liquid flow through the propelling zone is important since only the upward ow vector along the vertical dimension of the chip mass is effective to provide a propelling pressure. The radial ow vector is completely ineffective in that regard. Thus, the liquid ow through the zone must be generally upward and parallel to the vertical axis of the chip column, if effective chip propelling is to be achieved without unnecessarily high liquid llow velocities.

To oper-ate the digester of FIG. 1 according to the present invention, it is simply necessary to provide generally axial turbulent liquid upilow throughout a propelling zone at the lower end of the chip mass of substantial length at least about one-half the diameter of the chip mass preferably by recirculation through the propelling zone, providing a propulsion factor of at least about 500. The position of the lower end of the compacted chip mass is controlled by varying the recirculation. 'I'he recirculation is increased if the lower end of the chip mass moves downwardly and decreased if the lower end of the chip mass moves upwardly, such being accomplished by sensor 51 and its controlled valve 53. Otherwise, the digester operates in a conventional manner, with uncooked chips entering through tangential inlet 16 and being discharged at outlet 24, the other liquid ows within the digester being at conventional values as may be needed for treatment of the mass of wood chips within the vessel.

As a first example encountered in practice for a preferred embodiment of the apparatus of the invention as shown in FIG. 1 consisting of a continuous digester containing a chip column 10 feet in diameter by 70 feet high, with a very short or no drainage zone and a minimum number of strainers, and fed with a furnish of chips made from sawmill wasteespecially from a friable species such as Douglas r, and if all normal sawdust and slivers formed during chipping and handling were included without screening and if further the countercurrent flow zones were omitted, thereby reducing the resistance to upwardly movement of the chip column, the invention could :be operated at a propulsion factor of and the How required in the propelling zone would be for a 40 propelling zone, or

for a 6' propelling zone.

This example represents about the minimum propelling factor situation according to the present invention.

Some examples will be encountered where the resistance to chip column movement is much higher and the resistance to flow much less.

For a second example, iff a furnish of typical coniferous 'wood chips is to be treated, which have been cut from logs, using a chipper knife setting of 3A", the chips having passed through a chip screen to remove fines which will pass a 3/16 mesh wire; in this case the propulsion factor required for the same vessel as in the first example will be about and is will be found that :19.4 g.p.m./sq. ft.

If on the other hand it is desired to devote less height to propulsion and more to treatment at the cost of a higher flow, for example a propelling zone height of 6 feet may be selected, and

If, in the above example, the drainage zone at the top were increased by 3 feet in height above the liquid level, that is, the liquid level were dropped 3 feet from its normal level, the required propulsion factor would increase by 4150. This would require a total flow of 59.5 g.p.m./sq. ft. Further, if countercurrent ow were as is known in the art in a 40 foot zone with typically a 2.5 g.p.m./sq. ft. flow, an additional propulsion factor of 357 would be needed. Extra strainers needed by such added zone requires an additional propulsion factor of 1595 for each strainer.

Referring to FIG. 2 of the drawing showing a second embodiment of the invention, as with FIG. l, the pressurized vessel 112 therein shown is generally cylindrical of the order of ten feet or `greater in diameter and one hundred feet in length so that it has a length to diameter ratio of at least one to four. Preferably, vessel 112, although of generally uniform crossasectional area, has a slight taper to provide a gradually upwardly increasing cross-sectional area, a taper of about inch of diameter per foot of length being suitable. A wood chip-liquid infeed is provided at the lower entrance end of vessel 112 in the form of a tangential inlet 116 to which is fed, by means of a pump 118, preferably of the type shown and described in Pat. No. 2,908,226, a wood chip-liquid mixture from a supply tank 120. At the upper end of the vessel 112 is provided a discharger for the pulped wood chips in the form of a scraper 122 which establishes a predetermined upper level of the Wood chip mass by discharging drained pulped wood chips through discharge outlet 124 to a discharge mechanism at the lower end of a downfiow pipe (not shown) such as is described in Pat. No. 3,206,356.

Within vessel 112, generally the central vertical axis thereof, are provided a series of three concentric injection pipes 141, 143 and 153 having outlets 142, 144 and 154, respectively, at successively higher levels. Also within vessel 112 are provided strainers 146 and 148, with their valves 152 and 151, respectively, together with said outlets establishing a plurality of zones within the vessel by advancing liquid for either cocurrent or countercurrent ow within said zones as desired. For example, as shown in the drawing, there is provided a lowermost chip compaction zone with a superposed impregnation and heating zone followed by a heating and cooking zone with co-current and countercurrent ow and a countercurrent washing zone with an uppermost drainage zone, the liquid flows being shown by the single arrows and the wood chip movement by the double arrows. A liquid level control 150 is provided near the upper end of vessel 112 t0 establish the liquid level below scraper 122 and the upper level of the wood chip mass as by operating valve 152 of upper strainer 148 to provide the chip drainage zone.

A chip mass control system is provided for controlling the position of the lower end of the chip mass normally spaced above the lower end of the vessel for compacting chips thereon continuously to establish and maintain said lower end at a desired level. The apparatus components of such system include a lower liquid strainer 130 spaced a substantial distance preferably of about one-half to one and one-half times the vessel cross-section diameter above inlet pipe 116 and below scraper 124 defining a chip cornpaction zone therebelow and a recirculation pump 132, together with connecting pipes 134 and 138 and valves 169 and 175, 'for controlled recirculation of liquid through pump 118 to inlet 116. A pair of sensors 162, 164 are provided near the lower end of vessel 112 to control the position of the lower end of the chip mass through controller 168 as by operating valve 169 to vary the ow of recirculating liquid and hence the rate of movement of the wood chip mass. Such sensor and control systems are more fully described in co-pending patent application Ser. No. 543,362.

In accordance with the pulse liow aspects of the present invention, a liquid pulse generator is provided for causing the rate of movement of the submergence liquid relatively to the wood chips in the mass to vary cyclically for propelling the entire columnar chip mass upwardly through the vessel. Preferably such pulse generator, as shown in FIG. 2, is in the form of an accumulator 170, connected through valve 172 to inlet 174 in the lower end of vessel 112, valve 172 being operated by a cycle timer 166. Accumulator 170 is connected to a suitable source of air pressure through valve 176 so that it is pressurized to a value somewhat higher than that at the' bottom of vessel 112. It is also connected to the recirculating liquid pipe 138 through branch line 139. The level of liquid in accumulator is maintained by means of level controller 178 and its controlled valve 176 connected to a suitable source of pressurized liquid and by throttling valve in recirculation pipe 138.

More specically as to the methods of the invention and the operation of the above described apparatus of FIG. 2 in accordance therewith, as mentioned above in connection with FIG. l-it has been established in the commercial operation of digesters of the type shown and described in said Pat. No. 2,878,116 and said application Ser. No. 508,531 that the wood chips are present therein in the form of a columnar mass of contacting and compacted but discrete wood chips through which liquid can ow more or less independently of the wood chips in the mass but subject to a flow resistance dueto the presence of the wood chips, so that the wood chips within the mass can be moved throughout the length of the vessel solely by pushing on the bottom end surface of the columnar mass within the vessel regardless of the direction of liquid flow therein.

In accordance with the pulse flow aspect of the invention, we have discovered that the entire compacted chip mass may be advanced uniformly by subjection, preferably throughout its entire submerged length, to cyclical liquid advancing pulses created in the submergence liquid. By so doing, non-uniform chip compaction throughout the length of the chip column is eliminated.

In general, the use of liquid advancing pulses according to the invention must be effective to cause the rate of movement of the submergence liquid relatively to the wood chips in the mass to vary cyclically between an advancing movement of the liquid at a rate at least not less, and preferably greater, than that of the alternating intervening movement Of the submergence liquid. In fact, as hereinafter more fully explained, it is desirable that such advancing liquid pulse flow take place in turbulent ow, with the intervening pulse in laminar flow. However, in considering the precise nature of the pulses, the effect of the vessel configuration on the resistance to movement of the compacted chip mass therein must be taken into consideration, and the most important of these considerations has to do with the amount and direction of taper of the vessel. Thus, neglecting the effects of strainers, continuous liquid flows, length of chip column out of submergence, and the like, an untapered vessel will have equal resistance to movement of the chip mass therein in either direction, whereas a tapered vessel will have a lower resistance to such movement in the direction of increasing crosssectional area than in the opposite direction. In the case of a vessel in which the vessel cross-section increases in the direction of advance of the wood chip mass, it does not appear to be essential in theory that the advancing pulse produce a greater speed of liquid movement than the intervening pulse, although it is desirable that it be at least as great and preferably greater since the amount of vessel taper is preferably limited in order that a preferred amount of chip compaction occur, which we have found does not occur if the taper much exceed about one inch in vessel cross-section per foot of length. In an untapered vessel, it appears to be essential that the advancing pulse produce a much greater rate of liquid movement than that provided by the intervening pulse.

The use of an asymmetric pulse has the further advantage of exploiting the difference between laminar and turbulent liquid flow for producing the advancing force within the chip mass. This result is made possible because, as explained above, the rate of fluid flow resistance increases much more rapidly with increased flow velocity within the turbulent flow region than it does within the laminar flow region. Thus, as may be seen in FIG. 4, within the region of laminar ow, the resistance to flow is directly proportional to the flow velocity, whereas within the turbulent flow region it increases as the power (n) of the ow, wherein n is greater than unity. The resistance to ow created by the presence of the columnar mass of chips on which the liquid pulse of present invention acts can be determined by measuring the dilference (AP) between the lluid pressure (P1) at the bottom of the chip column and the fluid pressure (P2) at the top of the chip column. With fluid flow increasing from zero, the resistance (AP) will rise proportionally throughout the laminar ilow region, and then rather abruptly change in a short transition region to a much more rapid rise as the flow enters the turbulent region. In this way, the beginning of the turbulent flow region for a specific chip mass can readily be determined so that a fluid llow well within it, that is beyond the transition region, can readily be selected. Since turbulent flow occurs within a compacted chip mass at far lower ilow rates than had heretofore been assumed, liquid propulsion of the entire columnar chip mass can be achieved using pulsed liquid flow within the chip mass at ow rates not too high to be provided by conventional means, yet high enough to be within the turbulent flow region. These flows create suitably high flow resistance within the columnar chip mass to move it as desired so that the compacted wood chips can be advanced upwardly throughout the entire length of the columnar mass maintained within the vessel. The propelling force so created is sufliciently high to move the chip mass even if there be countercurrent liquid ilow throughout portions of the wood chip mass and a wood chip drainage zone at the upper end of said mass. The ability to provide such a highly desirable multiple zone upilow system with its inherent economics in heating, chemical retention and chip drainage is a particularly important feature of the present invention.

The wood chip advancing force of the present invention is generally proportional to the difference between the advancing pulse liquid ilow rate (raised to the power n) and the intervening liquid flow rate (raised to the power n). This difference will be more pronounced if only the advancing pulse be in turbulent ow, where n is greater than about 1.5, usually within the range of about 1.5 to 2.5, as is preferred. Although the actual difference depends greatly on the porosity of the chip mass and so varies depending ou the chip size, compressibility and the amount of sawdust or slivers included, the value needed for a particular digester can be determined from the characteristics and dimensions of the chip mass treatment zones malntained therein, the condition of such zones as to non-submergence or submergence and the forces created because of factors such as the presence of strainers and fluid ow forces cocurrent or countercurrent.

To operate the digester of FIG. 2 according to the invention, it is simply necessary to provide lluid pulses by cyclically opening and shutting valve 172 at a frequency of preferably about l to 30 cycles per minute that is 60 to 1800 cycles per hour but at least about 5 to lO cycles per hour as by valve timer 166, for example, an open time of about one second and a closed time of about tive seconds, resulting in discharging the accumulator through height Ah to produce a rapid height change (AH) in the level of submergence liquid in vessel 112 of about one to three inches followed by a slower restoring of its level as the accumulator is recharged by liquid pumped through branch line 139 by pump 132. These pulses act preferably in axial liquid upilow throughout the submerged portion of the chip mass to cause the rate of movement of the submergence liquid relatively to the wood chips in said mass to vary cyclically between an advancing movement of the liquid and an intervening movement of the liquid alternately to raise and lower the upper level of said submergence liquid through the limited distance AH.

The advancing movement of the liquid through the entire submerged portion of the chip mass is at a rate preferably in turbulent flow, while the intervening movement is in laminar ilow, providing a wood chip advancing movement, when opposed by the resistance within said vessel to advancing movement of said mass, substantially 10 greater than any wood chip retreating movement produced by the intervening liquid movement through said chip mass, when opposed by the resistance within said vessel to retreating movement of said mass. Such resistance in the case of the preferred tapered vessel of increasing cross-section in the direction of Wood chip advance is so much greater than it is in the opposite direction of chip mass movement that the chip mass advances upwardly in stepwise manner without intervening retreating steps.

The position L of the lower end of the compacted chip mass is controlled by varying the recirculation through pipes 134 and 138. To accomplish this result, the recirculation is increased if the lower end of the chip mass moves downwardly and is decreased if the lower end of the chip mass moves upwardly, such being controlled by sensors 162 and 164 and valve 169. Alternately, the bottom level L of the chip mass may be controlled by varying the pulse flow rate, as by increasing the accumulator air pressure, or the pulse frequency, for example.

Otherwise, the digester operates in a conventional manner, with uncooked chips entering through tangential inlet 116 and being discharged at outlet 124, the other liquid ows within the digester being at conventional values as may be needed for treatment of the wood chips within the vessel.

The invention, especially in its pulse aspect, is also useful in downflow digesters, since the forces created by its use are not limited merely by the force of gravity acting on the chips, but may be several times that amount. This makes possible the operation of multiple zone downow digesters having several strainers and countercurrent ow zones without danger of stopping the downward advance of the chip mass.

To that end, in FIG. 3 is shown a pressurized downow vessel 212 generally cylindrical of the order of ten feet or greater in diameter and about one hundred feet in length so that it has a length-to-diameter ratio of at least one to four. Preferably, vessel 212, although of generally uniform cross-sectional area, has a slight taperer to provide a gradually downwardly increasing cross-sectional area, although such is not as important as it is with upow vessels. A wood chip-liquid infeed is provided at the upper entrance end of vessel 212 in the form of a central inlet 214 to which drained wood chips are fed from the upper end of an upwardly inclined tube 215 by a screw feeder 216 therein. A pressurized wood chip-liquid mixture is fed to the upper side of the lower end of tube 215 by a suitable pump 218 connected to a wood chip supply tank 220 having a variable speed wood chip inlet conveyor 219 and a cooking liquor inlet pipe 221 with a control valve 223. A recirculation system is provided for tube 215 including a recirculation pipe 236 connected at one end to said tube and at its other end to tank 220, with throttling valve 238. A steam inlet 239 is provided at the upper end of tube 215. The liquid level in tank 220 is sensed by sensing element 225 for operation of throttling valve 238. At the lower end of vessel 212 is provided a discharger in the form of an agitator 222 which agitates the treated wood chips for discharge through discharge outlet 224 to a blow valve 226 cyclically operated in accordance with the invention as is hereinafter more fully described.

Within vessel 212, generally along the central vertical axis thereof, are provided a series of three concentric injection pipes 241, 243 and 253 having outlets 242, 244, and 254, respectively, at successively lower levels. Also within vessel 212 are provided strainers 246, 248 and 250 with their valves 247, 249 and 251, respectively, together with said outlets establishing a plurality of zones within the vessel by advancing liquid for either co-current or countercurrent ow within said zones as desired. For example, as shown in the drawing, there is provided an uppermost chip compaction zone folowed by an impregnation and heating zone, a heating and cooking zone with countercurrent ow and a countercurrent washing zone,

the liquid ows being shown by the single arrows and the wood chip movement by the double arrows.

A wood chip mass control system is provided for controlling the position of the upper end of the chip mass normally spaced below the upper end of the vessel to establish and maintain said upper end at a desired level. To accomplish this, a wood chip level sensing element 258 is provided near the upper end of vessel 212 to establish the upper level of the wood chip mass by controlling the speed of variable speed wood chip inlet conveyor 219 and hence the rate of wood chip feed into vessel 212.

There is also provided a liquid level control system for establishing and maintaining a desired liquid level below the top of vessel 212, providing a steam accumulation zone 230 thereabove by means of a fluid inlet pipe 261 and control valve 262 at the bottom of vessel 212. These are supplied with fluid, for example, from the filtrate tank 265 of a subsequent vacuum filter pulp washer by means of a high pressure pump 263. A liquid level sensing element 260 is provided to establish, by controlling valve 262, the desired liquid level below the top of vessel 212 defining the lower end of upper accumulator zone 230.

In accordance with the pulse ow aspects of the present invention, a liquid pulse generator is provided for causing the rate of movement of the submergence liquid relatively to the wood chips in the mass to vary cyclically for propelling the entire columnar chip mass downwardly through the vessel. Preferably, such pulse generator includes, as shown in FIG. 3, accumulator zone 230, connected to a suitable source of steam pressure through steam inlet 239 and a cycle timer 232 operating blow valve 226. As explained above, the level of liquid in vessel 212 defining the bottom of accumulator zone 230 is maintained by means of liquid level sensing element controller 260 and its controlled valve 262.

To operate the digester of FIG. 3 according to the invention, it is simply necessary to produce iluid pulses by cyclically opening and shutting blow valve 226 at a frequency of preferably about 1 to 30 cycles per minute but preferably about 5 to 10 cycles per minute as by valve timer 232, for example, an open time of about one second and a Iclosed time of about ve seconds, resulting in accumulator zone producing a rapid height change (AH) in the level of submergence liquid in vessel 212 of about one to three inches followed by a slower restoring of its level. These pulses act preferably in axial liquid upow throughout the submerged portion of the chip mass to cause the rate of movement of the submergence liquid relatively to the wood chips in said mass to vary cyclically between an advancing movement of the liquid in a downward direction and an intervening movement of the liquid alternately to raise and lower the upper level of said submergence liquid through the limited distance AH, in general as explained above with reference to FIG. 2, but in a downow rather than an upflow direction of chip movement.

With the novel pulse propulsion of FIGS. 2 and 3 of the invention, the advancing pulse of liquid acts on the submerged portion of the columnar mass of compacted wood chips throughout its entire length. Thus, if vessel resistance tends to compact one section of the mass to a greater extent than another, the pressure drop through such section will increase. This will increase the propulsive force in that section which will in turn act to decrease the packing. Conversely, if a section of the mass becomes loosely packed, the propulsive force will decrease, since if the liquid ows freely through loose chips, no propulsive force will be created. As a result, a loose section of the mass will not be advanced until the following chips in the mass advance to recompact such loose section. If, in actual operation of the digesters of FIGS. 2 or 3, any unusual resistance of movement of the chip mass is encountered, such as from a partial plug of chips clinging to a strainer, the amplitude or rate of the advancing pulse can be increased to provide additional liquid movement as needed to free the plug.

Still other modifications of the invention within the spirit thereof and the scope of the appended claims will be apparent to those skilled in the art.

What is claimed is:

1. A method of continuously pulping wood chips having a dimension greater than 1A6 inch in an elongated upow digester vessel comprising the steps of feeding substantially continuously said wood chips into said digester vessel adjacent the lower end thereof to maintain a columnar mass of compacted wood chips therein treating the wood chips in liquid submergence in a digesting liquid maintaining a predetermined level of wood chips at the upper end of said columnar mass establishing the upper level of said submergence liquid below said predetermined level generating liquid propelling pulses at a frequency of about 5 to 1800 cycles per hour effective to cause the rate of movement of said submergence liquid relative to said wood chips in said mass to vary cyclically to produce a cyclic advancing movement of said liquid at a turbulent liquid ow rate at least not less than that of the alternating intervening movement of said liquid, said advancing movement of said liquid providing the sole wood chip propelling force which is effective to advance the wood chips upwardly throughout the length of said columnar mass in opposition to a resistance created within said vessel and alternately to raise and lower the upper level of said submergence liquid through a limited distance of at least about 1 to 3 inches, and

substantially continuously discharging said wood chips from the upper end of said column at the upper end of said vessel.

2. A method as claimed in claim 1 further comprising the step of draining said wood chips at the upper end of said columnar mass out of liquid submergence in a drainage zone.

3. In a process as claimed in claim 1 wherein said advancing movement of said liquid is in turbulent ow and said intervening movement is in laminar flow.

References Cited i UNITED STATES PATENTS 2,999,538 9/1961 Hillmann 162-237 3,193,444 7/1965 Benjamin 162-237 X 3,303,088 2/1967 Gessner 162-19 3,200,032 8/ 1965 Richter et al 162-237 X FOREIGN PATENTS 60,797 12/ 1923 Sweden.

HOWARD R. CAINE, Primary Examiner U.S. Cl. X.R. 162-57 

